1. Field of the Invention
This invention relates to a method for forming a film with which a film can be deposited on a semiconductor wafer or the like.
2. Background of the Invention
In general, when a semiconductor integrated circuit is manufactured, a film-forming process and a pattern-etching process are repeated in turn to a silicon substrate such as a semiconductor wafer in order to form many desired devices.
A barrier metal is used just below a circuit layer that can electrically connect respective devices with each other in order to prevent a mutual diffusion between a substrate of Si and a circuit material of the circuit layer, and/or in order to prevent a detachment with respect to a ground layer. The barrier metal has to be a material having a low electrical resistance and a superior corrosion resistance. For example, a metal material having a high melting point such as a TiN film may be often used as the barrier metal.
In general, the barrier metal consisting of the TiN film may be formed by a thermal CVD by using TiCl4 gas as a high-melting-point metal compound gas and NH3 gas as a reducing gas.
In addition, in general, a pre-coated film consisting of a same kind of film as a film to be deposited i.e. a TiN film in the case is formed in advance on a surface of a stage for placing the semiconductor wafer in a film-forming unit that can conduct such a film-forming process, in order to maintain a thermal uniformity within a surface of the wafer and prevent a metal contamination caused by metal elements included in the stage. The pre-coated film is removed every when a cleaning process is conducted to the film-forming unit (for example, after a predetermined number of the wafers are processed). Thus, every after the cleaning process, as a pre-process, the thin pre-coated film is deposited on the surface of the stage.
The film-forming unit conducts the film-forming process to a semiconductor wafer that has been processed in a previous step of the film-forming process. Thus, the film-forming process is not always continuously conducted to a plurality of semiconductor wafers in the film-forming unit. For example, after the film-forming process is successively conducted one by one to twenty five semiconductor wafers that can be accommodated in one cassette, the film-forming unit may stay in an idling state for a time till a next cassette is conveyed and set. After the next cassette that accommodates preprocessed semiconductor wafers is conveyed and set, the film-forming unit may be back in the fully operating state. Then, the film-forming process is successively conducted one by one to twenty five semiconductor wafers in order to form the film. As described above, in general, the idling state and the fully operating state are repeated in turn.
In the above case, the time for the idling state may be long and/or short according to the completion state of the previous step of the film-forming process. For example, the time may be 10 minutes to 1 hour or more.
FIG. 8 shows an example of timing chart of the above conventional film-forming method. In the case, after the film-forming process is successively conducted to twenty-five semiconductor wafers in the film-forming unit, the film-forming unit stays in an idling state for 1 hour. After the idling, the film-forming process is successively conducted to next twenty-five semiconductor wafers, again. In the case, a process temperature of the film-forming state is for example 680xc2x0 C. but a waiting temperature of the idling state is reduced to for example 550xc2x0 C. in order to save energy. In addition, during the idling state, in order to prepare to resume the next film-forming process, an inert gas such as N2 gas is introduced into a processing container in the film-forming unit that can be brought into vacuum. Thus, it is prevented that an impure gas comes into the processing container or the like and that an environment in the processing container is changed or fluctuated.
As described above, in order to prevent the change of the environment in the processing container or the like, during the idling state, the N2 gas is continuously introduced. However, when the film-forming process is resumed and successively conducted to for example twenty-five semiconductor wafers again after the idling, a thickness of a film formed on only a first semiconductor wafer may tend to be not uniform. That is, the thickness of the film on the first wafer may be thicker or thinner than a desired thickness. In addition, repeatablity of the thickness of the film on the first wafer is not good and uniformity of the thickness within the film on the first wafer is not good. Thus, a dummy wafer is often used as the first semiconductor wafer just after resuming the film-forming process. The above problem happens only in the first semiconductor wafer just after resuming the film-forming process after the idling. The problem may also cause a low yield.
It may be thought that deterioration of the repeatability of the thickness of the film, especially on the first wafer after resuming the film-forming process, may be caused by a or more gases that have adhered to an inside wall of a pipe for the gases or an inside wall of the processing container. However, in addition, the thickness of the film on the first wafer may be thicker or thinner based on a condition of the pre-coated film on the stage or the like in each of film-forming units or even in the same film-forming unit. At present, although causes of the above problem have not been solved sufficiently, it has been required to prevent the above problem.
This invention is intended to solve the above problems. The object of this invention is to provide a method for forming a film that can improve repeatablity and uniformity with respect to a thickness of the film formed on a first object to be processed just after resuming a film-forming process after a long idling state.
The inventor has studied the repeatability of the thickness of the film eagerly and carefully. As a result, with respect to the film-forming process for the first object to be processed when resuming the film-forming process after the long idling state, the inventor has found that the repeatability can be improved by introducing only one of a material gas and a reducing gas for a short time as a pre-process just before the film-forming process.
That is, the invention is a method for forming a film comprising; a setting step for putting an object to be processed in a processing container that can be brought into a vacuum; a film-forming step for introducing both of a high-melting-point metal composition gas and a reducing gas into the processing container in order to deposit a predetermined film onto a surface of the object to be processed, subsequently to the setting step; and a pre-flowing step for introducing only one of the high-melting-point metal composition gas and the reducing gas into the processing container for a predetermined time, before the setting step.
According to the invention, atmosphere in the processing container can be maintained at substantially the same state as during the continuous film-forming process. Thus, repeatablity of a thickness of the film and uniformity of the thickness within the film can be improved.
The invention is more effective if the method further includes an idling step during which neither the high-melting-point metal composition gas nor the reducing gas is introduced into the processing container for a predetermined time, just before the pre-flowing step.
In the case, preferably, a high-melting-point metal composition gas is introduced into the processing container in the pre-flowing step, when a thickness of a film deposited on a first object to be processed in the just subsequent film-forming step tends to be thinner than a thickness of a film deposited on each of the subsequent objects to be processed if neither the high-melting-point metal composition gas nor the reducing gas are introduced in the pre-flowing step and a plurality of objects to be processed are processed in the subsequent film-forming step.
Alternatively, preferably, a reducing gas is introduced into the processing container in the pre-flowing step, when a thickness of a film deposited on a first object to be processed in the just subsequent film-forming step tends to be thicker than a thickness of a film deposited on each of the subsequent objects to be processed if neither the high-melting-point metal composition gas nor the reducing gas are introduced in the pre-flowing step and a plurality of objects to be processed are processed in the subsequent film-forming step.
The invention is also more effective if the method further includes an idling step during which said only one of the high-melting-point metal composition gas and the reducing gas is not introduced into the processing container for a predetermined time, just before the pre-flowing step.
As an example of the invention, a plurality of objects to be processed are put in the processing container in turn in the setting step, and a predetermined film is deposited on each of the plurality of objects to be processed successively in the film-forming step.
In the case, a high-melting-point metal composition gas is preferably introduced into the processing container in the pre-flowing step, when a thickness of a film deposited on a first object to be processed in the just subsequent film-forming step tends to be thinner than a thickness of a film deposited on each of the subsequent objects to be processed if neither the high-melting-point metal composition gas nor the reducing gas are introduced in the pre-flowing step.
Alternatively, a reducing gas is preferably introduced into the processing container in the pre-flowing step, when a thickness of a film deposited on a first object to be processed in the just subsequent film-forming step tends to be thicker than a thickness of a film deposited on each of the subsequent objects to be processed if neither the high-melting-point metal composition gas nor the reducing gas are introduced in the pre-flowing step.
In the former case, only the high-melting-point metal composition gas may be introduced into the processing container for example for 5 seconds or more in the pre-flowing step. In the latter case, only the reducing gas is introduced into the processing container for example for 10 seconds or more in the pre-flowing step.
In addition, the former manner is effective if the method includes an idling step during which the high-melting-point metal composition gas is not introduced into the processing container for a predetermined time, just before the pre-flowing step. The latter manner is effective if the method includes an idling step during which the reducing gas is not introduced into the processing container for a predetermined time, just before the pre-flowing step.
The high-melting-point metal composition gas may be for example TiCl4 gas, and the reducing gas may be for example NH3 gas.